Influenza infections are a leading cause of death in the United States and worldwide, with 50 000 of deaths occurring annually in the United states alone. Although many patients succumb to the primary viral infection, a significant number of influenza-related deaths are attributable to the development of secondary bacterial infections. Why the host is more susceptible to bacterial infections post-influenza is poorly understood. A better understanding of how the host responds to sequential influenza + bacterial infection is necessary in order to develop therapies to improve outcomes. Using a murine model of sequential pulmonary influenza and methicillin-resistant Staphylococcus aureus (MRSA) or Streptococcus pneumoniae infection, we have found that these mice have decreased survival and impaired anti-bacterial responses. We also showed that host responses to post-viral pneumonia are characterized by impaired macrophage autophagy responses, and impaired macrophage phagocytosis and killing of bacteria in the lungs. These findings correlate with significant upregulation of interferon (IFN)? and upregulation of miR 155 when compared to mice infected with either pathogen alone. Thus, we hypothesize that impaired innate immunity against MRSA in the setting of post-influenza pneumonia is due to IFN?-induced upregulation of miR 155 which in turn blocks protective autophagy responses and prevents bacterial phagocytosis and killing. We will address these goals with the following specific aims. Aim 1) To determine the role of IFN? in regulating miR 155 expression, host defense, cytokine production, and macrophage autophagy following infection with influenza alone, MRSA/Streptococcus pneumoniae alone or sequential infection. This aim will characterize these outcomes in single and sequential infections, will test responses in chimeric mice generated using wild-type and IFN? receptor-/- mice and will test a therapeutic approach to block IFN? using a neutralizing mAb. Aim 2) To determine whether the impaired host defense noted during post-viral pneumonia is due to a failure of lung macrophages to upregulate protective autophagy responses This aim will determine whether autophagy is impaired in lung macrophages post- sequential infection compared to single infection, whether this is correlated with miR 155 inhibition of DAPK1 and whether enhancement of autophagy using rapamycin/Tat Beclin-1 can improve host defense by increasing bacterial phagocytosis and killing. Aim 3) To identify the mechanism that miR-155 uses to regulate macrophage recruitment, autophagy and to impair the anti-bacterial host-defense. This aim will characterize expression of miR 155 during post-viral pneumonia and explore inhibition of autophagy during post-viral pneumonia as a result of miR 155 inhibition of DAPK1. The ability of miR 155 to regulate bacterial phagocytosis and killing also will be explored. Additionally, transgenic mice will be generated using floxed miR155 and CD11b-Cre mice enabling cell type specific removal of miR155 from macrophages further providing mechanistic insight regarding regulation of autophagy by miR 155 expression on macrophages.